Interpretive Summary: Almonds, pistachios, and walnuts are among the most valuable crops grown in California. However, a large portion of the crop is exported and must comply with strict regulatory levels limiting aflatoxin contamination, a carcinogen produced by the mold Aspergillus flavus. Naturally occurring compounds found in almonds, walnuts, and pistachios were tested and found to decrease aflatoxin contamination by up to 99.8%. These compounds, including antioxidants and tannins, were effective at decreasing aflatoxin contamination even under conditions that normally stimulate Aspergillus flavus to produce high levels of aflatoxin, such as oxidative or drought stress. Increasing the levels of these naturally occurring compounds in tree nuts has the potential to significantly reduce aflatoxin contamination in tree nuts and allow for a larger share of these exported crops to comply with the aflatoxin regulatory standards of the foreign markets.

Technical Abstract:
Almonds, pistachios, and walnuts grown in California have an aggregate value of over $3.3 billion, with a large proportion of the crop being exported. However, these tree nuts can be subject to contamination by aflatoxins, metabolites produced primarily by Aspergillus flavus and parasiticus, and importing nations often impose strict regulatory levels limiting the levels of aflatoxins in the interest of food safety. Endogenous natural products from tree nuts, including hydrolyzable tannins, flavonoids, and phenolic acids, were found to decrease aflatoxin production by as much as 99.8%. Oxidative and drought stress conditions were shown to stimulate high levels of aflatoxin production by Aspergillus flavus that were maintained over time. Tannic, gallic, and caffeic acid all worked to counteract the stress conditions and decreased the production of aflatoxin by Aspergillus flavus. Increasing antioxidant natural products in tree nuts may restrict the ability of Aspergillus flavus to biosynthesize aflatoxin, thus achieving conformity with regulatory limits.